The precipitation of inorganic salts, such as calcium carbonate, calcium sulfate, barium sulfate, or strontium sulfate, from aqueous fluids to form scale is a persistent and common problem encountered in many field operations for the recovery of hydrocarbons from subterranean formations. In particular, scale growth in aqueous formation fluids and subsequent undesirable deposition of the scale in the formation as well as production equipment and tubing is typically caused by incompatible fluids which are commingled in situ during field operations, especially enhanced oil recovery (EOR) operations involving waterflooding or a water drive.
By definition, aqueous fluids are deemed incompatible if each fluid contains distinct ions which form a precipitate that deposits as a scale when the fluids are commingled. A common example of two incompatible fluids is a connate water or brine containing barium, calcium, or strontium cations and an EOR injection water containing sulfate anions. Offshore operations often involve the injection of large volumes of sea water containing relatively high concentrations of sulfate anions into subterranean formations containing brine with relatively high concentrations of the above-listed cations. Upon mixing of the sea water and formation brine in situ, precipitation of barium, calcium or strontium sulfate scale occurs in the formation as well as in the subsurface and surface production tubing and equipment. Scale growth is most detrimental in the near production well bore environment of the formation, i.e., within about 2 or 3 meters of the production well bore.
The presence of carbon dioxide in the formation can also exacerbate scaling problems. Carbon dioxide is frequently introduced into the formation during EOR processes resulting in absorption of carbon dioxide into the connate water. Some formation brines, such as those found in the North Sea, may also naturally contain relatively high concentrations of carbon dioxide. When pressure is reduced in a formation containing fluids with a relatively high carbon dioxide concentration, such as during hydrocarbon production, the carbon dioxide flashes to the gas phase, thereby increasing the pH of the aqueous fluids and causing the growth of calcium carbonate scale in the near production well bore environment and in the production tubing and equipment.
Conventional methods for removing scale deposited in a subterranean formation or production equipment and tubing are both costly and ineffective. Chemical removal methods are known wherein a chemical agent is repeatedly injected into the affected formation, equipment, or tubing to attack the scale. Such methods are, however, relatively expensive and, thus, impractical. Scale is also removable using various mechanical devices, such as impact or cavitation jets. The mechanical devices are usually employed downhole after first killing the well. Mechanical scale removal methods are, however, likewise relatively impractical, being time-consuming, inefficient, and potentially hazardous, particularly where a radioactive precipitate, such as radium sulfate, is present in the scale being removed.
Preventative methods for inhibiting the growth and deposition of scale have been advanced as a more preferred approach to the problem of scale deposition. Conventional commercial scale inhibitors primarily comprise polyelectrolytes, such as polycarboxylates or polyphosphonates. However, the effectiveness of such polyelectrolyte scale inhibitors depends to a large part on the degree of ionization of the inhibitor at the pH value of the connate water. At relatively low pH values, e.g., about 6 or less, the effectiveness of conventional polyelectrolyte scale inhibitors for inhibiting the growth of barium, calcium, or strontium sulfate scale significantly decreases. In addition, conventional polyelectrolyte scale inhibitors dissolve calcium carbonate scale, thereby undesireably increasing the calcium ion concentration in the formation fluids and causing precipitation of the inhibitors themselves.
In view of the problems set forth above, it is apparent that a need exists for a scale inhibitor having utility in a subterranean formation, which effectively inhibits the growth of scale therein. A need is particularly acute for a scale inhibitor which effectively inhibits the growth of inorganic sulfate scale, such as barium sulfate, in relatively low pH environments of about 6 or less.
Accordingly, it is an object of the present invention to provide a scale inhibitor which will effectively inhibit the growth of scale, particularly inorganic sulfates such as barium sulfate, in an aqueous fluid present in or produced from a subterranean formation. It is another object of the present invention to provide a scale inhibitor which does not promote the dissolution of carbonate scale to the same extent as conventional polyelectrolyte scale inhibitors when placed in a subterranean formation. It is a further object of the present invention to provide a process for inhibiting scale growth, particularly inorganic sulfate scale such as barium sulfate, in aqueous fluids having a pH of about 6 or less which are present in or produced from a subterranean formation.